1. Field of the Invention
This invention relates to the field of conveyor chains and in particular to a so-called "acceleration accumulating" conveyor chain of the type wherein conveyed articles can be carried on the chain while moving at other than the chain speed, e.g., the articles can move in excess of the chain speed, or the articles can remain stationary as the chain carrying the articles passes underneath.
2. Prior Art
A typical conventional acceleration accumulating chain (hereinafter simply termed a "chain") is shown in FIG. 7. The chain 10 comprises bushings 13, having article-carrying rollers 11 of relatively larger diameter and running rollers 12 of relatively smaller diameter, coaxial with one another and with their respective bushings. Connecting pins 14 extend through the bushings 13, and inner link plates 15, 15 as well as outer link plates 16, 16 are disposed at both ends of the connecting pins 14. The running rollers 12 can support the chain on rails (not shown in FIG. 7) as articles rest on article rollers 11 and the chain is carried longitudinally along a chain run by the connected parts.
Such an arrangement requires relative rotation of the connected parts of the links in order to allow the article rollers to rotate with passage of the articles and to allow the running rollers to rotate with passage of the chain over the rails. In the known chain 10 as shown in FIG. 7, "play" is intentionally imparted to the connected parts in addition to the clearance necessary to allow relative rotation, namely by means of a clearance between the connecting pin 14 and its mating aperture of the inner link plate 15. This play allows the running rollers 12, 12 to roll positively along the rails, maintaining contact even if the alignment of the chain (and in particular the connecting pins) relative to the rails is disturbed.
This solution to the problems of permitting relative rotation of the parts and maintaining desired roller alignment has adverse consequences with respect to wear on the chain, noise generated by the chain in use, and the maintenance requirements for dealing with these intentionally loose fitting parts.
Loads on this form of chain are concentrated on the relatively small contact area between the smaller connecting pin 14 and the larger apertures 17 of the inner link plates 15. A high bearing pressure per unit area occurs at the point of contact. Wear at the contact area therefore tends to be higher than would accrue over a larger contact area, and wear causes elongation of the chain. As a result, the optimum allowable load on the conventional chain is relatively low.
Noises due to friction are generated by the known chain, especially when the chain engages a sprocket (not shown). A periodic supply of lubricating oil can help to reduce friction and noise, however, such lubrication is not always possible or desirable, for example in a clean room area where dirt and dust must be suppressed. In such environments, noise is a problem and the chain wears and periodically must be replaced. Relatively high levels of noise are also produced by the chain running on the rails, particularly if the bushings are made of steel, as is usually preferred.
Another type of known chain is shown in FIG. 8, and is characterized by certain modifications intended to cope with the problems of the conventional chain shown in FIG. 7. Chain 20 as shown in FIG. 8 has elongated bushings 23, which are press fit to attach rigidly to inner link plates 25, 25. The connecting pins 24 have a diameter d.sub.1 which is smaller than the inner diameter D of the bushings 23. This structure allows the connecting pins 24 to contact the inner surface 23A of the bushings 23 all along their length, at least when the connecting pins are parallel to their respective bushings, resulting in a decreased contact pressure per unit area as compared to the conventional chain of FIG. 7. The clearance for chain play in this embodiment is between the connecting pin and the inside of bushing 23, the latter being attached to the inner link plate, and the former being attached to the outer link plate.
However, if during assembly or thereafter the connecting pin 24 and the bushing 23 are not precisely parallel, twisting forces or torsions are generated, tending to misalign links of the chain. These torsions may not be absorbed within the clearances between the pins 24 and the bushings 23. Misalignment may cause the running rollers 22 to lose contact with the rail, thereby defeating the acceleration accumulating function which is a basic object of this type of chain. Inasmuch as the diameter of the pins is relatively small, it is often difficult to attain the desired degree of accuracy in parallel alignment of the pins and bushings for all the links in a chain when manufactured. Furthermore, the arrangement is apt to wear. Accordingly, satisfactory solutions to the problems of the conventional chain remain to be achieved.